Question

Which of the following must be known to calculate the molarity of a salt solution (there may be more than one answer)? a. the mass of salt added b. the molar mass of the salt c. the volume of water added d. the total volume of the solution Explain

Short answer

To calculate the molarity of a salt solution, we need to know the mass of salt added (option a), the molar mass of the salt (option b), and the total volume of the solution (option d). Option c, the volume of water added, is not required. The molarity can be calculated using the formula: \(Molarity = \frac{moles\ of\ solute}{volume\ of\ solution\ in\ liters}\).

Step-by-step solution

Analyze Option a – Mass of salt added

The amount of salt added to the solution is needed to calculate the molarity. We need to determine the number of moles of salt, and to do that, we need to know the mass of salt added to the solution. So, option a is required.

Analyze Option b – Molar mass of the salt

To determine the moles of solute in the solution, we also need to know the molar mass of the salt. Moles can be calculated by dividing the mass of salt by the molar mass of salt. So, option b is required.

Analyze Option c – Volume of water added

The volume of water added to the salt is irrelevant when calculating the molarity. We need to know the total volume of the resulting solution, not just the volume of water added. So, option c is not required.

Analyze Option d – Total volume of the solution

Knowing the total volume of the solution is crucial for calculating molarity. As molarity is defined as the number of moles of solute per liter of solution, the total volume of the solution must be known. So, option d is required.

Conclusion

Calculating the molarity of a salt solution requires knowing the mass of the salt added (option a), the molar mass of the salt (option b), and the total volume of the solution (option d). The volume of water added (option c) is not needed. Therefore, the correct choices are options a, b, and d.

Key concepts

Mass of Solute

When calculating the molarity of a solution, one of the essential components to consider is the mass of the solute, in this case, the salt added. The mass of the solute is the amount, typically in grams, of the substance that gets dissolved in the liquid to create the solution. Knowing this mass is vital because it serves as the starting point for finding the number of moles of the solute in the solution.

To find the number of moles, you will use the equation:

• \( ext{moles of solute} = \frac{ ext{mass of solute}}{ ext{molar mass of solute}} \)

The calculation begins with knowing exactly how much of the salt you have in terms of mass. Without this information, it would be impossible to proceed further in calculating molarity, as the entire calculation hinges on knowing the amount of solute you are working with.

Molar Mass

Molar mass is another critical concept needed in the calculation of molarity. Molar mass represents the mass of one mole of a given substance and is usually expressed in grams per mole (g/mol). It serves as a conversion factor from the mass of the solute to the number of moles of the solute.
When you have the mass of your solute, you will need to divide this by the molar mass to determine how many moles of the solute are present:

• \( ext{moles of solute} = \frac{ ext{mass of solute}}{ ext{molar mass}} \)

Knowing the molar mass of the particular salt you are working with allows you to transform the measurable mass into the more useful moles when determining molarity. Every chemical compound has a specific molar mass, which you can typically find on the periodic table for individual elements or through reputable chemical references for compounds.

Total Volume of Solution

The total volume of the solution is indispensable in the molarity calculation. Molarity is defined as the number of moles of solute per liter of solution, and thus having a precise measurement of the solution's total volume is crucial.
This measure refers to the sum of the volume of the solute and solvent combined after they have been mixed. Importantly, do not confuse this with just the volume of the solvent, such as water added. We need the complete volume post-mixing, as this impacts the concentration:

• \( ext{Molarity (M)} = \frac{ ext{moles of solute}}{ ext{total volume of solution in liters}} \)

Being precise with the measurement of the total volume ensures that the calculated molarity accurately reflects the solution's concentration, which is crucial for consistency and reproducibility in scientific experiments and industrial applications.